Add chamber-$(VERSION).sha256sums generation (#126)

Makefile

@@ -26,13 +26,27 @@ release: gh-release clean dist
 	--name chamber-$(VERSION)-linux-amd64 \
 	--file dist/chamber-$(VERSION)-linux-amd64
 
+	github-release upload \
+	--security-token $$GH_LOGIN \
+	--user segmentio \
+	--repo chamber \
+	--tag $(VERSION) \
+	--name chamber-$(VERSION).sha256sums \
+	--file dist/chamber-$(VERSION).sha256sums
+
 clean:
 	rm -rf ./dist
 
 dist:
 	mkdir dist
 	GOOS=darwin GOARCH=amd64 CGO_ENABLED=0 go build $(LDFLAGS) -o dist/chamber-$(VERSION)-darwin-amd64
 	GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build $(LDFLAGS) -o dist/chamber-$(VERSION)-linux-amd64
+	@which sha256sum 2>&1 > /dev/null || ( \
+		echo 'missing sha256sum; install on MacOS with `brew install coreutils && ln -s $$(which gsha256sum) /usr/local/bin/sha256sum`' ; \
+		exit 1; \
+	)
+	cd dist && \
+		sha256sum chamber-$(VERSION)-* > chamber-$(VERSION).sha256sums
 
 gh-release:
 	go get -u github.com/aktau/github-release